Summary

Create a multipart encoding of the specified node. The returned binary
node can be passed to xdmp:http-post.
The manifest is modeled after the
manifest that is passed to zip:create,
with the headers element being
the same as is described for xdmp:http-get
allowing users to add arbitrary
headers to each part. If a content-type header is not specified for a part,
it will be determined if possible from the content.

There should be one part element for each node in the content sequence.

Each part also has an optional options node to control how xml or text
will be serialized. The two options are the same as for
xdmp:save.